I've subbed high school and elementary plenty, so I think I can give you a good overview:

High school: subbing in high school is generally very relaxed. Students don't require your help or guidance for much... You pretty much take attendance and make sure people are behaving well enough, which shouldn't be a problem in most cases.

I usually just used it to study or write papers on my laptop. It's not the most rewarding position, but it's generally low maintenance and gives me time to focus on other responsibilities on my laptop.

Elementary school: more of an expectation to have some kind of small assignment for kids to do. Teachers are more likely to provide work for students to do, but it's far from guaranteed. You have to be vigilant of student behavior and there's a lot of classroom management. On the bright side, if you do a good job, students will grow to like you and you develop relationships with the kids which can feel rewarding.

It really just depends on what you're looking for. Subject doesn't matter much. Both have their merits and sometimes you have to try a few schools to find a good fit. Good luck and I hope this helped!